In a small bowl, stir together masa harina and salt. Add hot water and oil - stir just until the dough comes together. Evenly divide the mixture into 24 tiny balls (about the size of the beans) - if you want, you can make these a little bigger as it is tedious to get 24 out of this small portion of dough. Add dumplings to the soup and cook until they begin to float - about 3-5 minutes. Stir in chopped cilantro and lime juice right before serving. Top each portion with about 1 tablespoon of the feta cheese once portioned out.